mTLS support for batch and custom http object

[jay0lee]

First stab at addressing #2440.

• mTLS logic in build_from_document such as choosing the mtlsRootUrl should be executed even if we pass a custom http object in.
• add an is_mtls bool attribute to Resource so we can make quick decisions about the state of mTLS later.
• ensure our batch URI is using the mTLS host when is_mtls == True

feedback welcome.